Electrical power generators using gas turbines as the motive power source are well known. The gas turbines used to power such generators typically have power outputs of from 1.6 megawatts to over 200 megawatts. In use, the temperature of the combustion gases in the combustor generally exceeds the melting point of the metal alloy from which the combustor is made, and particularly efficient cooling of the combustor walls is a necessary requirement, to prevent melting of the combustor.
Four basic methods of cooling combustor walls, or walls of other hot components, are in common use. These are as follows:
Film Cooling.
Cooling air flows through one or more rows of holes in the wall and spreads over the hot side of the wall as a thin film of cooling air. Film cooling may be applied over the entire inner wall of a combustor. Disadvantages are that the efficiency of combustion is impaired by the passage of cooling air to the inside of the combustor. Some of that air inevitably mixes with the combustion gases and lowers the combustion reaction zone temperature, quenching the reaction and increasing pollutant emissions. There is a loss of efficiency, and it is in general necessary to maintain a low mass flow of cooling air in order to maximize the air available for combustion and thereby reduce primary pollutant emissions.
Impingement Cooling.
A perforated cooling jacket is provided around the combustor wall, to define therebetween a heat exchange chamber. Adequate heat exchange is created by establishing a very rapid flow of cooling air or other gaseous coolant through the perforations, so producing small jets of coolant which impinge upon the outside of the combustor wall. An advantage of impingement cooling is that it can be used with a relatively low mass flow of air with correspondingly high pressure loss, so maximizing the air available for combustion. The air is commonly reintroduced downstream of the reaction zone, thereby also reducing quenching pollutants. Hence this type of cooling outperforms film cooling where pollutant emissions are critical. Furthermore, correct design can ensure low sensitivity to manufacturing and other tolerances.
Convection Cooling.
A cooling jacket is provided around the combustor, to define a heat exchange chamber between the jacket and the combustor outer wall. The heat exchange chamber has a hot wall, which is the outer wall of the combustor, and a cool wall, which is the wall of the cooling jacket. Adequate heat exchange between the wall and coolant is created purely by establishing a rapid flow of cooling air through the chamber. The dimensions of the system are relatively small, so requiring high accuracy components. This is costly and the cooling performance is sensitive to manufacturing or installation tolerances and movement during operation. The system offers the advantage of relatively low pressure loss and so the cooling air can be re-used for combustion without excessive efficiency penalty, thereby avoiding the pollutant effects associated with the first two types above.
Enhanced Convection Cooling.
A cooling jacket is provided around the combustor, to define therebetween a heat exchange chamber. The hot wall is provided with fins extending into the heat exchange chamber, and a current of cooling air is passed over those fins. Thermal transfer between the cooling air and the fins provides the cooling necessary. Typical dimensions are larger than equivalent plain convection cooling, reducing the sensitivity to tolerances. However, a major disadvantage of this heat exchange structure is that the fins have to be provided on the hot combustor wall, which is typically made from a high specification and expensive alloy. The cost of forming that alloy into a finned surface is correspondingly high. Furthermore, the thermal gradients induced by non-uniform thickness of the combustor wall are detrimental to the operating life of the combustor, as are the stress-concentrating properties of the fins.